Environ. Sci. Technol. 2007, 41, 6313-6314
Response to Comment on “Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors” We write in response to the comment from Leng et al. (1) on our paper regarding the photocatalytic oxidation mechanism of arsenite (2). Since we first reported the proposition that the photocatalytic oxidation (PCO) of As(III) is mainly mediated by the superoxide (3, 4), several studies followed to address the interesting issue of PCO mechanism of As(III) to confirm or to dispute our claim (5-8). The mechanism of the heterogeneous photocatalytic reaction is very complex and often ambiguous in many cases. Even the same experimental data can be interpreted in different ways and the mechanism itself may vary depending on the experimental condition, which explains why many debates on the photocatalytic mechanism fail to generate a clear consensus. In this case, Leng et al. particularly raised an issue on the validity of the photoelectrochemical data we provided. Our replies to three main issues are as follows. First, on the basis of the eqs 5 and 6 they used to express the photocurrent generation, they questioned the premise that the presence of As(III) as an electron donating species should increase the photoanodic current on the illuminated TiO2 electrode and claimed that the relative change (positive or negative) of the photocurrent upon spiking As(III) cannot indicate whether As(III) adsorbed on TiO2 is oxidized or reduced. The general mathematical expression for the photocurrent production at the steady-state can be written as eq 1
Jph ) q(
∑ k [h ][D ] - ∑ k [e ][A ] ) +
j
i
-
j s
i
i s
(1)
i
where Jph is the photocurrent density (A‚cm-2), q is the charge of an electron, kj is the rate constant for the hole transfer to an electron donor (Dj) on the surface, and ki is the rate constant for the electron transfer to an electron acceptor (Ai) on the surface. Equation 1 indicates that the addition of any electron donor should increase Jph. The main electron donor species in this case are As(III), surface hydroxyl group, and adsorbed water molecule. If we want to compare the photocurrents before and after the addition of As(III), that should be ∆Jph ) kj[h+][As(III)] if other species remain unaffected. Leng et al. argued that the surface hydroxyl group concentration ([OH-]surf) on TiO2 may be changed when As(III) is present and that the photocurrent change should be ∆Jph ) [h+](k1[As(III)] + k2[OH-]surf ′ - k2[OH-]surf). The point is whether the term (k2[OH-]surf ′ - k2[OH-]surf) is negligible or not. We can estimate it from the As(V) spike test we carried out in the previous study (2). Since As(V) is neither an electron donor nor an electron acceptor, any photocurrent change upon spiking As(V) should be ascribed to (k2[OH-]surf ′ k2[OH-]surf). The adsorption of As(V) on TiO2 is about twice as high as that of As(III) under the identical condition (4). Therefore, the adsorption of As(V) should deplete more surface hydroxyl groups than As(III) and should induce more change in the photoanodic current according to the logic of 10.1021/es0707456 CCC: $37.00 Published on Web 07/31/2007
2007 American Chemical Society
Leng et al. However, we observed no change of the photoanodic current when As(V) was spiked (2). Therefore, we can ′ - k2[OH-]surf) is negligible conclude that the term (k2[OH-]surf in our test condition. Second, Leng et al. carried out photoelectrochemical experiments (similar to ours) in the presence of either formate or benzoic acid and found that the relative change of the photoanodic current is opposite between the two cases. The spiking of As(III) in the presence of formate dropped the photocurrent (as we observed) but further increase of [As(III)] in the successive spikings increased the photocurrent. On the other hand, the spiking of As(III) in the presence of benzoic acid increased the photocurrent on the contrary. This is an interesting point we did not realize. We also confirmed their results in our laboratory. However, this does not contradict our proposed mechanism. Our explanation is that the mechanism changes when the reaction condition changes. The relative change of the photoanodic current is related with the PCO mechanism of As(III) that depends on the reaction condition. For example, in the commented paper (2), we showed that the relative photocurrent change is reversed in the direction when alternative electron acceptors such as polyoxometalate and Cu2+ are present because their presence changes the PCO mechanism. The As(III) PCO mechanism also depends on the concentration of arsenite. The change in the photoanodic current (∆Jph ) JAs(III) - J0ph) ph upon spiking As(III) changes the sign from negative to positive when [As(III)]spike reaches the mM range (data not shown). This implies that the PCO mechanism is changed at higher arsenite concentration.
As(III) + hVB+ (or surface-bound OH•) f As(IV) (2) As(IV) + eCB- f As(III)
(3)
As(IV) + As(IV) f As(V) + As(III)
(4)
As(IV) f As(V) + eCB-
(5)
The key point of our proposed mechanism is that reactions 2 and 3 take place successively with forming a null chargerecombining cycle. However, when the concentration of As(III) increases, the bimolecular disproportionation of As(IV) (reaction 4) or direct electron injection into the conduction band (reaction 5) may compete with reaction 3. In such a case, As(III) can be oxidized by holes and the photoanodic current should increase. However, this mechanism has little practical significance since such a high concentration of As(III) is irrelevant to the environmental condition. In any case, the observed Jph is a result of the balance between reaction 3 versus reactions 4 and 5. Higher [As(IV)] enhances not only reaction 3 (negative contribution to Jph) but also reactions 4 and 5 (positive contribution to Jph). The case of benzoic acid in which the photocurrent increased upon spiking As(III) can be understood similarly. The presence of organic substrates, the oxidized intermediates, or the organic radicals may interfere with reactions 2-5 by serving as an electron acceptor, an electron donor, a complexing agent, or a competing adsorbate. That is, their presence can influence the PCO mechanism in many different ways. We carried out the additional photocurrent experiments in the presence of other organic substrates (e.g., acetate, 4-chlorophenol, and t-butyl alcohol) and the photocurrent responses were all different. Therefore, the relative change of the photoanodic current upon spiking As(III) is meaningful only when there VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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is no interfering organic. Overall, the photocurrent change obtained in the presence of organic substrates (e.g., formate and benzoic acid) seems to be too complex to be taken as a simple indicator for As(III) oxidation. Finally, Leng et al. questioned how the superoxides are generated if the conduction band electrons are consumed by the As(III)/As(IV) recombining cycles. What we say is not that all electron-hole pairs are consumed completely by the null cycle, but that most adsorbed As(IV) species that are produced from reaction 2 subsequently react with the conduction band electrons. These are different statements. That is, most charge-pairs are recombined through the null cycle but there are some charge-pairs that survive this recombination cycle. The observation that the photocatalytic degradation rate of formate was markedly reduced but not completely inhibited in the presence of As(III) supports that this is the case (2). The remaining electrons react with O2 to generate superoxides while the remaining holes generate OH radicals that recombine to H2O2. H2O2 should be further decomposed by reacting with holes or electrons. As we discussed (2), the estimated photostationary concentration of superoxide is much reduced in the presence of As(III) (10-11 vs 10-6 M), but this small concentration of superoxide is high enough to explain the observed PCO rate. In summary, the superoxide mechanism we consistently support is applied to the normal condition that contains only dissolved oxygen and As(III) in water. Our message is that this main superoxide-driven oxidation mechanism can be changed when there is an alteration in the photocatalytic reaction condition. What makes the confusion is this: how we probe the reaction mechanism may change the mechanism itself! Probing As(III) oxidation mechanism has been done by adding some probe compounds (e.g., formate, POM, Ag+) on the basis of the assumption that it does not influence the reaction mechanism. However, it does in some cases. Finally, we would like to stress that the photocurrent data that Leng et al. questioned are only a small part of the evidence on which our proposed mechanism is based
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although they seem to argue that our mechanism is based solely on the photoelectrochemical data. Our studies on the PCO mechanism of As(III), which were published in three papers (2-4), contain many different kinds of experimental data that all agree with the proposed mechanism.
Literature Cited (1) Leng, W. H.; Cheng, X. F.; Zhang, J. Q.; Cao, C. N. Comment on “Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors”. Environ. Sci. Technol. 2007, 41, 6311-6312. (2) Ryu, J.; Choi, W. Photocatalytic Oxidation of Arsenite on TiO2: Understanding the Controversial Oxidation Mechanism Involving Superoxides and the Effect of Alternative Electron Acceptors. Environ. Sci. Technol. 2006, 40, 7034-7039. (3) Lee, H.; Choi, W. Photocatalytic oxidation of arsenite in TiO2 suspension: Kinetics and mechanisms. Environ. Sci. Technol. 2002, 36, 3872-3878. (4) Ryu, J.; Choi, W. Effects of TiO2 surface modifications on photocatalytic oxidation of arsenite: The role of superoxides. Environ. Sci. Technol. 2004, 38, 2928-2933. (5) Ferguson, M. A.; Hoffmann, M. R.; Hering, J. G. TiO2photocatalyzed As(III) oxidation in aqueous suspensions: Reaction kinetics and effects of adsorption. Environ. Sci. Technol. 2005, 39, 1880-1886. (6) Xu, T.; Kamat, P. V.; O’Shea, K. E. Mechanistic evaluation of arsenite oxidation in TiO2 assisted photocatalysis. J. Phys. Chem. A 2005, 109, 9070-9075. (7) Dutta, P. K.; Pehkonen, S. O.; Sharma, V. K.; Ray, A. K. Photocatalytic oxidation of Arsenic(III): Evidence of hydroxyl radicals. Environ. Sci. Technol. 2005, 39, 1827-1834. (8) Yoon, S.-H.; Lee, J. H. Oxidation mechanism of As(III) in the UV/TiO2 system: Evidence for a direct hole oxidation mechanism. Environ. Sci. Technol. 2005, 39, 9695-9701.
Jungho Ryu and Wonyong Choi* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea ES0707456
